gH and plant deficiencies question

Out of the tap, our well water has a kH of 161.1 ppm & gH 107.4 ppm (API test). One would think this means pretty good levels of calcium, magnesium and carbonate. I'm curious if anyone knows if the API tests are remotely reliable or can be thrown off by high levels of silicate or other dissolved inorganic compounds? (I'm curious because after observing indications of nutrient deficiencies in plants over the past several months, I suspect that the water is low in magnesium and high in silicates.)

I had my water company come out and test for all that, but with you being on a well, not sure how you may be able to find out that answer.